1. Field of the Invention
This invention relates to an improved roof anchor which is at least partially retained in a rock formation by resin bonding material and which has an axially adjustable resin retaining washer that is positionable along the shaft of the roof anchor to retain the resin at the end of the roof anchor and to exert a compressive force on the resin before it sets.
2. Description of the Prior Art
It is well known in the art of mine roof support to tension roof anchors in bore holes drilled in the mine roof to reinforce the unsupported rock formation above the roof. Conventionally, a hole is bored through the roof into the rock formation. The end of the roof anchor is anchored in the rock formation by either engagement of an expansion shell on the end of the anchor with the rock formation or by adhesively bonding the anchor with a resin bonding material to the rock formation surrounding the bore hole or by using a combination of a mechanical expansion shell and resin bonding material. When resin bonding material is utilized, it penetrates the surrounding rock formation to adhesively unite the rock strata and to firmly hold the roof anchor in position within the bore hole. The resin mixture fills the annular area between the bore hole wall and the shaft of the roof anchor.
U.S. Pat. No. 4,419,805 and U.S. Pat. No. 4,413,930 are examples of mine roof anchors utilizing a combination of an expansion shell and a resin bonding material to retain the roof anchor within the rock strata. These patents disclose rigid resin retaining washers which are axially fixed to the shaft of the roof anchor and which may not be adjusted after the roof anchor leaves the manufacturing site and is delivered to the mine site.
U.S. Pat. No. 4,162,133 also shows a roof anchor which is retained within a rock strata by both a mechanical expansion anchor and resin bonding material. This patent discloses a rigid resin retaining washer that is supported on the shaft of the roof anchor by ears that are pinched into the shaft of the roof anchor by ears that are pinched into the shaft of the roof anchor in a fixed position. The rigid resin supporting washer of this patent is not axially adjustable since the position of the ears pinched into the shaft of the roof anchor will determine the position that the rigid washer assumes when the resin comes into contact with it.
We have found that by providing an axially adjustable rigid resin retaining washer we can accurately coordinate the annular area available for the resin to occupy with the amount of resin that is utilized in the roof anchor system so that when the roof anchor is installed, the upward thrust of the roof anchor will exert a hydraulic force on the resin bonding material to confine it within a restricted annular area at the end of the roof anchor and to cause the resin bonding material to be forcefully driven into the cracks and crevices on the inside wall of the bore hole and into the surrounding rock formation to more solidly lock the roof anchor within the rock formation.